Recent deadly mass shootings in these US cities have at least one thing in common: the AR-15. The weapon was discovered again at the scene of Sunday's deadly shooting at a Tennessee Waffle House, as suspect Travis Reinking remains on the run.

This weapon has become increasingly popular in the US, especially since the 1994 federal weapons ban expired in 2004, and has been used in many other mass shootings around the country. Not just the three listed above.

To understand how and why this has happened, we put together a historical overview of the weapon and spoke with David Chipman, a senior policy analyst at Giffords and former special agent with the Bureau of Alcohol, Tobacco, Firearms, and Explosives.

The National Rifle Association did not respond to our request for comment.

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The AR in AR-15 stands for Armalite Rifle — not assault rifle.

A US soldier wields an M-16 during the Vietnam War in 1965.
Associated Press

In the mid-1950s, the US Army asked a gun-manufacturing company called Armalite to develop a smaller version of the AR-10 to replace the M-1 Garand, which had been widely used in World War II and the Korean War.

The result was the AR-15.

But Armalite then sold the design to Colt, which in turn began selling the weapon to Pentagon. In 1962, the US Department of Defense changed the name of the AR-15 to the M-16.

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In 1963, Colt began marketing the AR-15 to the American public as a "superb hunting partner."

Colt

While it was still legal for gun dealers to sell automatic weapons until the 1986 Firearm Owners Protection Act, which banned new automatic weapons, these first Colt AR-15s were semi-automatic weapons.

An automatic continuously fires when the trigger is held down, whereas the operator must continuously pull the trigger to repeatedly fire a semi-automatic weapon.

However, to this day, civilians can still own automatic weapons that were grandfathered in before 1986.

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And, even then, the AR-15 was incredibly lethal.

An AR-15 on display.
Joe Raedle/Getty Images

It shoots a .223 Caliber or 5.56 mm round at roughly 3,300 feet per second, which is about three times the muzzle velocity of a typical Glock pistol.

The AR-15's effective firing range is also more than 1,300 feet at the least, whereas a typical Glock's firing range is just over 160 feet.

Chipman, the senior policy analyst at Giffords and former ATF special agent, told Business Insider that the AR-15 is so powerful that they weren't allowed to carry it during indoor raids because the rounds travel so fast that they could penetrate a victim, then a wall, then a bystander through that room.

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But US civilians rarely purchased semi-automatic weapons until the 1989 school shooting in Stockton, California, which killed five people and wounded 29 more.

Police examine the covered body of a shooting victim at the Cleveland Elementary School in Stockton, Calif., Jan. 17, 1989.
Associated Press

"Before Stockton, most people didn't even know you could buy those guns," Chris Bartocci, author of the book "Black Rifle II," told CNN in December 2017.

"This stuff has been around forever; this is not new technology," Bartocci said, adding that the media surrounding the incident helped glorify semi-automatic weapons.

Not only did Chinese AK-47s, which the gunman in that incident used, sales go up, but AR-15 sales did as well, according to CJ Chivers.

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In 1994, Congress passed the federal assault weapons ban, which lasted for 10 years and included a number of AR-15 models.

Associated Press

The 1994 assault weapons ban, however, was complicated. It banned 18 kinds of firearms, such as certain AK-47 and AR-15 models, but it didn't ban all semiautomatic weapons.

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But when the ban expired in 2004, all models of AR-15s were once again readily available.